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Date:   Thu, 1 Sep 2022 15:37:14 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Uros Bizjak <ubizjak@gmail.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] KVM/VMX: Do not declare vmread_error asmlinkage
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On Wed, Aug 31, 2022, Uros Bizjak wrote:
> On Wed, Aug 17, 2022 at 5:58 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > +PeterZ
> >
> > On Wed, Aug 17, 2022, Uros Bizjak wrote:
> > > There is no need to declare vmread_error asmlinkage, its arguments
> > > can be passed via registers for both, 32-bit and 64-bit targets.
> > > Function argument registers are considered call-clobbered registers,
> > > they are saved in the trampoline just before the function call and
> > > restored afterwards.
> >
> > I'm officially confused.  What's the purpose of asmlinkage when used in the kernel?
> > Is it some historical wart that's no longer truly necessary and only causes pain?
> >
> > When I wrote this code, I thought that the intent was that it should be applied to
> > any and all asm => C function calls.  But that's obviously not required as there
> > are multiple instances of asm code calling C functions without annotations of any
> > kind.
> 
> It is the other way around. As written in coding-style.rst:
> 
> Large, non-trivial assembly functions should go in .S files, with corresponding
> C prototypes defined in C header files.  The C prototypes for assembly
> functions should use ``asmlinkage``.
> 
> So, prototypes for *assembly functions* should use asmlinkage.

I gotta imagine that documentation is stale.  I don't understand why asmlinkage
would be a one-way thing.

> That said, asmlinkage for i386 just switches ABI to the default
> stack-passing ABI. However, we are calling assembly files, so the
> argument handling in the callee is totally under our control and there
> is no need to switch ABIs. It looks to me that besides syscalls,
> asmlinkage is and should be used only for a large imported body of asm
> functions that use standard stack-passing ABI (e.g. x86 crypto and
> math-emu functions), otherwise it is just a burden to push and pop
> registers to/from stack for no apparent benefit.

Yeah, this is what I'm confused about.  Unless there's something we're missing,
we should update the docs to clarify when asmlinkage is actually needed.

> > And vmread_error() isn't the only case where asmlinkage appears to be a burden, e.g.
> > schedule_tail_wrapper() => schedule_tail() seems to exist purely to deal with the
> > side affect of asmlinkage generating -regparm=0 on 32-bit kernels.
> 
> schedule_tail is external to the x86 arch directory, and for some
> reason marked asmlinkage. So, the call from asm must follow asmlinkage
> ABI.

Ahhh, it's a common helper that's called from assembly on other architectures.
That makes sense.

Thanks much!